I guess right now it'd have to be the Rays. We very well could have taken 3/4 against the Tigers but for a blown save by Bailey and a blown call on Nava's catch in RF. And those games were at DET. Right now, Price and Moore look insanely tough, although Moore blew up last time out against the Yankees.
Regardless, if the Sox get a top of the rotation starter (which it is looking more and more like they will), I think they can match up with any team in terms of pitching, particularly with Lester pitching the way he is. For example, if we get my boy Lee, a playoff rotation of Lee - Buchholz - Lester would be an outrageous rotation to face, especially in a short series. Add in Lackey in the 4 slot in a 7 game series, that's tough to beat right there.